In the Dover Edition score, there's a page for "Intstruments and Voices". On that page it says the following: "Tenor [Tenor-Stimme, Ten-St.] . . Alto [Alt-Stimme, Alt-St.] or Bartione". It's not clear to me if they printed what Mahler wrote down, or that it's just the work of an editor. Regardless, this is exactly how it's notated on that page.
Mahler never heard a single note of this work, much less performed it. Obviously, I think that he would prefer that it be sung by two contrasting singers. However, I do think he would be flattered that somebody thought so highly of the work, that they would want to perform it all themselves. That's hardly an insult. Personally, I think Kaufmann has done a good of providing a contrasting style between the two groups of songs. He's done a good job of following the dynamics marked on the page as well.
Not to try to justify Kaufmann, but time and time again, Mahler chose singers who could deliver the text in a convincing style - while also accurately following the rhythms and dynamics on the page - to those who simply had a beautiful voice. The rest is up to our judgement as listeners.
I like this recording and I am keeping it. But that's more for the playing of the VPO and Sony's generally fine sonics (far better than DG in the Musikverein). But I'm also not really bothered by what Kaufmann has done here. In fact, I really prefer him to someone who over-sings the dynamics in "Der Abschied", which happens all the time. It is not an extended aria for Mezzo or Baritone, with the orchestra providing little more than accompaniment. If anything, it's the other way around. The orchestration and the dynamics on the page prove that point.